We recently bought a new projector. It seems that we now have to make services in 16:9 ratio. As a result we deleted the old services that were in 4:3. It seems that none of the songs stayed in in the program to reuse.
It is not a huge deal, since we just started but is the idea to never delete a service?
If you delete a presentation, the idea is that you no longer want that presentation or the items contained in it, so they will be removed from the "Reuse Items" list.
Additionally, it's too late now, but for future reference: you can easily switch any existing presentation from 4:3 to 16:9 and back again by changing the aspect ratio in the upper toolbar. The slides will be automatically resized.